Vacuum toilet systems have been known for many years. British Patent Specification No. 902,871 published Aug. 9, 1962 (Liljendahl) discloses an early vacuum toilet system. The system disclosed in U.S. Pat. No. 3,239,849 issued Mar. 15, 1966 (Liljendahl) may be regarded as the forerunner of the modern vacuum toilet system. Variations on the system disclosed in these early publications can be found in U.S. Pat. Nos. 4,275,470 (Badger et al), 4,357,719 (Badger et al) and 3,996,628 (Mollerstedt).
The modern vacuum toilet system comprises a waste-receiving bowl, a container that can be maintained under a pressure that is substantially lower than that in the interior of the waste-receiving bowl, and a sewer pipe that connects the waste-receiving bowl to the container. It is also known to interpose a sluice device between the sewer pipe and the container, and maintain the sewer pipe under reduced pressure while the container remains at atmospheric pressure. The sewer pipe may also be put under reduced pressure only for the time needed to transport waste to the container. In all cases, a discharge valve is provided for controlling passage of material from the waste-receiving bowl into the sewer pipe. The valve may be, for example, a gate valve as shown in U.S. Pat. No. 3,239,849, or a pinch valve as shown in U.S. Pat. No. 3,984,080 (Varis et al). The system also comprises a source of rinse water and a rinse water valve for controlling introduction of rinse water into the bowl.
In operation of a conventional vacuum toilet system, a user initiates the flushing operation, e.g. by actuating a flush control switch, and the sequence of steps that constitutes the flushing operation is performed automatically. The rinse valve is opened to rinse the bowl and the discharge valve is opened to empty the bowl. Waste material in the waste-receiving bowl is propelled by atmospheric pressure past the discharge valve and into the sewer pipe, because the sewer pipe is under partial vacuum. After a predetermined time that is estimated to be sufficient to remove substantially all the waste material from the bowl, the discharge valve is closed. Depending on the length of the sewer pipe, the waste material may be transported all the way to the container before the discharge valve is closed, but this is not essential. The rinse water valve may remain open for a short time after the discharge valve has closed, if it is desired to provide a pool of clean water in the bottom of the bowl.
If a solid object falls in the waste-receiving bowl, it may interfere with proper operation of the discharge valve, e.g. by preventing the valve from closing. Nearly all obstructions of this kind can be cleared by repeating the flushing operation. Hitherto, it has been necessary for the user or an attendant to actuate the flush control switch manually in order to initiate a second flushing operation so as to remove the obstruction. This is inconvenient, and moreover a user of the vacuum toilet system might not be familiar with the correct procedure.
It has for many years been conventional to use recirculating toilet systems in aircraft. This is subject to the disadvantage that recirculating toilet systems employ recirculating liquids that are corrosive, and accordingly leakage from the system may enable the corrosive liquid to contact structural members of the aircraft, resulting in the structural integrity of the aircraft being impaired. Recirculating systems are used in aircraft because of the relatively large amount of liquid required to transport the waste material in a gravity toilet system. Vacuum toilet systems do not lend themselves to recirculation, because of the large pressure difference between the downstream side of the discharge valve and the upstream side of the rinse water valve. However, because vacuum toilet systems rely on vacuum for removal of the waste material from the bowl, the amount of rinse liquid that is needed in a vacuum toilet system is much smaller than the amount of rinse liquid required in a gravity toilet system. Consequently, non-recirculating vacuum toilet systems employing water as rinse liquid are attractive for use in aircraft.
The aircraft vacuum toilet system described in U.S. Pat. No. 4,275,470 is subject to the disadvantage that the movable element of the discharge valve is a flexible diaphragm positioned at the exterior of a curve in a pipe that connects the waste-receiving bowl to the sewer pipe. If a solid object falls in the waste-receiving bowl and the discharge valve is subsequently opened, the solid object is propelled at quite a high speed through the discharge pipe, and the flexible membrane is precisely in the trajectory of the solid object. Accordingly, the diaphragm is liable to be struck and damaged by the solid object. It is, therefore, desirable to avoid the use of flexible membranes in the discharge valve of a vacuum toilet system, particularly in the case of an aircraft vacuum toilet system where safety is critical.
A gate valve having a displaceably-reciprocating closure member is not suitable for use as the discharge valve in a vacuum toilet system, because solid material that might enter the valve housing is liable to be trapped between the leading edge of the closure member and a wall of the housing and ultimately interfere with the proper operation of the valve.
Conventionally, the discharge valve of a vacuum toilet is operated by means of the pressure difference provided by the vacuum. Electrical operation of the valve is feasible as well, but use of an electrically-operated valve and a solenoidoperated rinse water valve in a vacuum toilet system that operates in accordance with the conventional sequence of steps is subject to the disadvantage that both the discharge valve motor and the rinse water valve solenoid draw current at the same time.